This invention relates to a skin protective agent which exhibits a water-barrier function to human skin including hair.
Skin exposed to severe external environment, together with aging, causes rough skin, which is also referred to as a dry skin. Dry skin, that is, the dryness of skin, is basically brought about by an inability of the skin""s normal function of regulating the moisture (water) content of the skin and other protection systems. The main role in these functions comes from water.
It has recently become clear that the water in the skin (stratum corneum) not only performs the moisturization as a simple physicochemical action for skin but also control""s the enzymatic activities in the skin metabolic system. There has been reported, for example, the NMF (Natural Moisturizing Factor) generation based on the degradation of filaggrin (Dermatology, 2000, 773 (1993)) or the desquamation of corneocytes due to the digestion of intercellular adhesive factors (intercellular lipids, desmosomes and the like) (Dermatology, 100, 510 (1993)). The reduction in enzyme activities and the imbalance of keratinization as set forth above are caused by an insufficient moisturization of the stratum corneum, that is, the reduction of a water-barrier level. Therefore, it is to be said that the water-barrier abilities are the essential function to maintain the healthy skin. Although the water-barrier function depends upon the skin surface lipids, the amino acids as the NMF, and intercellular lipids of the stratum corneum, etc., it has been clarified that the intercellular lipids contributes highly to the barrier function (J. Inv. Dermatol., 84, 282 (1985)). The intercellular lipids mainly consist of ceramides, fatty acids, cholesterol and cholesterol sulfate (J. Inv. Dermatol., 88, 709 (1987)), and the total and respective levels of these lipids may be influenced by diet, age, race, environment (for example, seasons) and other factors (the 17th IFSCC, 2, 865 (1992)).
To provide a water-barrier property, there has been hitherto suggested the incorporation of various water retention agents into cosmetics.
There has been recently suggested a skin protective agent which comprises a racemate of ceramides, which is a mixture of the erythro and threo types of ceramides (See, Japanese Patent Kokai Nos. 327563/1992 and 165690/1995). Ceramides form the erythro and threotypes based on the conformation of the hydroxy group and the secondary amines and the respective types may take the different steric configurations represented by xe2x80x9cSxe2x80x9d and xe2x80x9cRxe2x80x9d.
The present inventors have made earnest studies to develop a skin protective agent providing an excellent water barrier benefit, and, as a result, have found that the erythro (2S,3R) type, an optically active species of ceramides, exerts a more remarkable excellent water-barrier function as compared with the recemate. These erythro (2S,3R) ceramides are available through synthetic route of manufacture, as described below.
It is, accordingly, a primary object of this invention to provide a skin protective agent which exhibits a remarkably excellent water-barrier property.